The present invention relates generally to Medium Attachment Units ("MAU") of a Local Area Network ("LAN") and more specifically to an improved MAU for implementation of communication protocols with a twisted pair cable medium for a LAN system.
FIG. 1 is a block diagram of a conventional Local Area Network ("LAN") 10 of a type proliferating in the computing market. These LANs permit a Data Terminal Equipment ("DTE") 12.sub.i, a computer or business machine which provides data in a digital form, to transfer data and control information with other DTEs 12.sub.i. Communication from a first DTE 12.sub.1 to a second DTE 12.sub.2 is implemented by use of Data Communications Equipment ("DCE") 14.sub.i, which provide functions required to establish, maintain and terminate a connection. DCE 14 provides whatever signal conversion or processing that is necessary or desirable.
There are two interfaces which are important to understand. These include a DTE/DCE interface 16.sub.i and a DCE/DCE interface 18, commonly referred to a transmission channel, or medium. For proper and reliable communication, a set of rules for communication between like processes, which provide a means of controlling information transfer between stations (DTEs) on a datalink, known as a protocol are implemented.
A popular protocol referred to as Carrier Sensing, Multiple Access, Collision Detection ("CSMA/CD") has been commercially successful. This protocol permits multiple stations to access a LAN system simultaneously. Each station, before transmitting, will sense for a carrier signal indicating that the network presently is being used to transmit a message. If it senses the carrier signal, transmission will not be initiated. It is possible, due to time delays in propagating a signal from a DTE 12.sub.i, that two transmissions will overlap. This overlap is referred to as a collision, which will be detected by DCEs 14.sub.i on the network. Upon detecting a collision, all transmissions will be terminated and DTEs 12.sub.i desiring to transmit will wait a random period of time before attempting to transmit again. This protocol is further defined in IEEE Standard 802.3, hereby expressly incorporated by reference for-all purposes, which sets forth requirements for the DTE/DCE interface 16.sub.i, referenced as an Attachment Unit Interface ("AUI"). The IEEE Standard 802.3 defines a system which is similar to Ethernet.RTM., a registered trademark of Xerox Corporation.
FIG. 2 is a block diagram of an example of a particular type of LAN 10 of FIG. 1, which implements the IEEE Standard 802.3. A plurality of nodes 20, each of which includes a System Interface Adapter ("SIA") 22, are the DTEs 12.sub.i of FIG. 1. SIA 22 is a encoder/decoder which translates information from Node 20 to a form required by the IEEE Standard 802.3. SIA 22 is coupled to a Medium Attachment Unit ("MAU") 26 by an AUI 30. MAU 26 and AUI 30 corresponds to DCE 14 and DTE/DCE interface 16, respectively, of FIG. 1.
Each MAU 26 must meet certain prespecified requirements, depending upon the type of medium to which it "translates" data to and from SIA 22. A standard identifying these prespecified requirements for MAU 26 is identified, for example, as 10 Base 5. This designation comes from three important physical layer (a model concerned with defining mechanical, electrical, functional and procedural characteristics of a physical link between two communicating devices) parameters. These parameters include transmission speed in megabits per second ("mbps"), whether baseband or broadband transmission is used and a segment length in hundreds of meters. Thus, 10 Base 5 identifies a physical layer which uses baseband transmission at 10 mbps at a length up to 500 meters, which are typical values for a coaxial cable medium. A draft standard, 10 Base T has been proposed which identifies standards for twisted pair cable medium.
AUI 30 may include a plurality of 1:1 coupling transformers 32 which provide isolation which are not always needed in particular applications. MAU 26 is coupled to a coaxial cable 34 which is used as the medium interconnecting the plurality of Nodes 20 of LAN 10. A repeater 36 is also provided which is used to amplify or regenerate signals passed in the system to compensate for losses. Repeater 36 will also resynchronize signals as necessary.
The LAN 10 illustrated in FIG. 2 may be expensive or difficult to implement because of the coaxial cable 34 which must be installed. In preexisting buildings and other structures, the expense and difficulty to install the cable increase. Many buildings have pre-installed unshielded twisted pair wires used for telephone services. By using these pre-existing lines in lieu of the coaxial cable, a significant savings in cost may be realized. As the reader will readily appreciate, the transmission of digital signals on these twisted pair lines is more difficult as the lines are susceptible to noise and attenuation. Additionally, preexisting wiring may use combinations of wires of differing gauges and have many different types of terminations and nodes which make transmission along those wires unpredictable.
An implementation of LAN 10 with twisted pair cable may be performed, however, by changing MAU 26 from an access unit for coaxial cable to an access unit for twisted pair cable. As compatibility of the IEEE Standard 802.3 is desired, other components, e.g., AUI 30 and SIA 22, remain exactly the same.
It is therefore another object of the present invention to implement a new and improved MAU which can properly interface an AUI to a twisted pair medium and maintain compatibility with the IEEE Standard 802.3 and draft standard 10 Base T. In implementing this new MAU, improvements and features have been incorporated which are not addressed by the 10 Base T standard. Portability of the improved MAU to networks which do not necessarily implement all current features of the IEEE Standard 802.3 must be maintained. It is therefore an object of the present invention to ensure that portability to pre-standard networks may be simply and efficiently ensured.
For example, a link test is implemented in the current standard as an active idle by which information may be exchanged among active devices on a network assuring each device that the network is operational. Some pre-standard systems do not send link pulses, therefore post standard devices attached on a network with them will not receive link pulses, indicating a link failure where none is present. A prior art solution to this problem is to provide an ability of new devices to totally inhibit both link pulse receive and link pulse transmit functions.